Grout line thickness will make a difference.
I start with the finish edge pieces that go around perimeter (ceiling too), all in a straight line. Hold them up and mark with pencil. (Have many pencils ready.) Use a level to see how square whole shower box is. Vertical and horizontal lines must be level and straight.
Hold up 2 12 x 12s in a corner and mark where true edge will be on other wall, allowing for thickness of thinset. The open area on each surface needs a vertical centerline next. Mark it. Now a matter of filling in as many full or near full pieces in both directions, across (very important) and up and down (less important if your not tiling to ceiling). Allow for your groutline spacing.
Idea is to get as near as possible to full size pieces on far left and far right rows. If it turns out to be a 'sliver' (very thin piece), rework plan until something works. If you're putting in decorative contrasting tiles and/or pencil pieces, plan and draw carefully on the wall. Much easier if not going to ceiling and decorative pieces are on horizontal plane only. Then first piece at floor level can be full piece IF shower base is perfectly level. Typically not the case.
